Refactor basic_fields allocator internals:

fix #857

* Use empty base optimization to store the allocator

* Rebind to a smaller aligned type to reduce waste

CHANGELOG.md

@@ -1,6 +1,7 @@
 Version 145:
 
 * Rename some detail functions
+* Refactor basic_fields allocator internals
 
 --------------------------------------------------------------------------------
 

include/boost/beast/http/fields.hpp

@@ -14,6 +14,7 @@
 #include <boost/beast/core/string_param.hpp>
 #include <boost/beast/core/string.hpp>
 #include <boost/beast/core/detail/allocator.hpp>
+#include <boost/beast/core/detail/empty_base_optimization.hpp>
 #include <boost/beast/http/field.hpp>
 #include <boost/asio/buffer.hpp>
 #include <boost/intrusive/list.hpp>
@@ -51,6 +52,9 @@ namespace http {
 */
 template<class Allocator>
 class basic_fields
+#ifndef BOOST_BEAST_DOXYGEN
+    : private beast::detail::empty_base_optimization<Allocator>
+#endif
 {
     friend class fields_test; // for `header`
 
@@ -260,7 +264,7 @@ public:
     allocator_type
     get_allocator() const
     {
-        return alloc_;
+        return this->member();
     }
 
     //--------------------------------------------------------------------------
@@ -681,12 +685,15 @@ private:
     template<class OtherAlloc>
     friend class basic_fields;
 
-    using base_alloc_type = typename
+    using align_type = typename
+        boost::type_with_alignment<alignof(value_type)>::type;
+
+    using rebind_type = typename
         beast::detail::allocator_traits<Allocator>::
-            template rebind_alloc<value_type>;
+            template rebind_alloc<align_type>;
 
     using alloc_traits =
-        beast::detail::allocator_traits<base_alloc_type>;
+        beast::detail::allocator_traits<rebind_type>;
 
     using size_type = typename
         beast::detail::allocator_traits<Allocator>::size_type;
@@ -736,7 +743,6 @@ private:
     void
     swap(basic_fields& other, std::false_type);
 
-    base_alloc_type alloc_;
     set_t set_;
     list_t list_;
     string_view method_;

include/boost/beast/http/impl/fields.ipp

@@ -350,14 +350,15 @@ basic_fields<Allocator>::
 template<class Allocator>
 basic_fields<Allocator>::
 basic_fields(Allocator const& alloc)
-    : alloc_(alloc)
+    : beast::detail::empty_base_optimization<Allocator>(alloc)
 {
 }
 
 template<class Allocator>
 basic_fields<Allocator>::
 basic_fields(basic_fields&& other)
-    : alloc_(std::move(other.alloc_))
+    : beast::detail::empty_base_optimization<Allocator>(
+        std::move(other.member()))
     , set_(std::move(other.set_))
     , list_(std::move(other.list_))
     , method_(other.method_)
@@ -370,9 +371,9 @@ basic_fields(basic_fields&& other)
 template<class Allocator>
 basic_fields<Allocator>::
 basic_fields(basic_fields&& other, Allocator const& alloc)
-    : alloc_(alloc)
+    : beast::detail::empty_base_optimization<Allocator>(alloc)
 {
-    if(alloc_ != other.alloc_)
+    if(this->member() != other.member())
     {
         copy_all(other);
         other.clear_all();
@@ -389,8 +390,8 @@ basic_fields(basic_fields&& other, Allocator const& alloc)
 template<class Allocator>
 basic_fields<Allocator>::
 basic_fields(basic_fields const& other)
-    : alloc_(alloc_traits::
-        select_on_container_copy_construction(other.alloc_))
+    : beast::detail::empty_base_optimization<Allocator>(alloc_traits::
+        select_on_container_copy_construction(other.member()))
 {
     copy_all(other);
 }
@@ -399,7 +400,7 @@ template<class Allocator>
 basic_fields<Allocator>::
 basic_fields(basic_fields const& other,
         Allocator const& alloc)
-    : alloc_(alloc)
+    : beast::detail::empty_base_optimization<Allocator>(alloc)
 {
     copy_all(other);
 }
@@ -417,7 +418,7 @@ template<class OtherAlloc>
 basic_fields<Allocator>::
 basic_fields(basic_fields<OtherAlloc> const& other,
         Allocator const& alloc)
-    : alloc_(alloc)
+    : beast::detail::empty_base_optimization<Allocator>(alloc)
 {
     copy_all(other);
 }
@@ -1014,13 +1015,13 @@ set_chunked_impl(bool value)
             // Workaround for https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56437
             std::string s;
         #else
-            using rebind_type =
+            using A =
                 typename beast::detail::allocator_traits<
                     Allocator>::template rebind_alloc<char>;
             std::basic_string<
                 char,
                 std::char_traits<char>,
-                rebind_type> s{rebind_type{alloc_}};
+                A> s{A{this->member()}};
         #endif
             s.reserve(it->value().size() + 9);
             s.append(it->value().data(), it->value().size());
@@ -1051,13 +1052,13 @@ set_chunked_impl(bool value)
         // Workaround for https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56437
         std::string s;
     #else
-        using rebind_type =
+        using A =
             typename beast::detail::allocator_traits<
                 Allocator>::template rebind_alloc<char>;
         std::basic_string<
             char,
             std::char_traits<char>,
-            rebind_type> s{rebind_type{alloc_}};
+            A> s{A{this->member()}};
     #endif
         s.reserve(it->value().size());
         detail::filter_token_list_last(s, it->value(),
@@ -1108,13 +1109,13 @@ set_keep_alive_impl(
         // Workaround for https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56437
         std::string s;
     #else
-        using rebind_type =
+        using A =
             typename beast::detail::allocator_traits<
                 Allocator>::template rebind_alloc<char>;
         std::basic_string<
             char,
             std::char_traits<char>,
-            rebind_type> s{rebind_type{alloc_}};
+            A> s{A{this->member()}};
     #endif
         s.reserve(value.size());
         detail::keep_alive_impl(
@@ -1148,13 +1149,14 @@ new_element(field name,
         static_cast<off_t>(sname.size() + 2);
     std::uint16_t const len =
         static_cast<off_t>(value.size());
-    auto const p = alloc_traits::allocate(alloc_,
-        1 + (off + len + 2 + sizeof(value_type) - 1) /
-            sizeof(value_type));
+    auto a = rebind_type{this->member()};
+    auto const p = alloc_traits::allocate(a,
+        (sizeof(value_type) + off + len + 2 + sizeof(align_type) - 1) /
+            sizeof(align_type));
     // VFALCO allocator can't call the constructor because its private
-    //alloc_traits::construct(alloc_, p, name, sname, value);
+    //alloc_traits::construct(a, p, name, sname, value);
     new(p) value_type{name, sname, value};
-    return *p;
+    return *reinterpret_cast<value_type*>(p);
 }
 
 template<class Allocator>
@@ -1162,10 +1164,14 @@ void
 basic_fields<Allocator>::
 delete_element(value_type& e)
 {
-    auto const n = 1 + (e.off_ + e.len_ + 2 +
-        sizeof(value_type) - 1) / sizeof(value_type);
-    alloc_traits::destroy(alloc_, &e);
-    alloc_traits::deallocate(alloc_, &e, n);
+    auto a = rebind_type{this->member()};
+    auto const n =
+        (sizeof(value_type) + e.off_ + e.len_ + 2 + sizeof(align_type) - 1) /
+            sizeof(align_type);
+    //alloc_traits::destroy(a, &e);
+    e.~value_type();
+    alloc_traits::deallocate(a,
+        reinterpret_cast<align_type*>(&e), n);
 }
 
 template<class Allocator>
@@ -1207,7 +1213,7 @@ realloc_string(string_view& dest, string_view s)
         return;
     auto a = typename beast::detail::allocator_traits<
         Allocator>::template rebind_alloc<
-            char>(alloc_);
+            char>(this->member());
     if(! dest.empty())
     {
         a.deallocate(const_cast<char*>(
@@ -1235,7 +1241,7 @@ realloc_target(
         return;
     auto a = typename beast::detail::allocator_traits<
         Allocator>::template rebind_alloc<
-            char>(alloc_);
+            char>(this->member());
     if(! dest.empty())
     {
         a.deallocate(const_cast<char*>(
@@ -1298,7 +1304,7 @@ move_assign(basic_fields& other, std::true_type)
     target_or_reason_ = other.target_or_reason_;
     other.method_.clear();
     other.target_or_reason_.clear();
-    alloc_ = other.alloc_;
+    this->member() = other.member();
 }
 
 template<class Allocator>
@@ -1308,7 +1314,7 @@ basic_fields<Allocator>::
 move_assign(basic_fields& other, std::false_type)
 {
     clear_all();
-    if(alloc_ != other.alloc_)
+    if(this->member() != other.member())
     {
         copy_all(other);
         other.clear_all();
@@ -1331,7 +1337,7 @@ basic_fields<Allocator>::
 copy_assign(basic_fields const& other, std::true_type)
 {
     clear_all();
-    alloc_ = other.alloc_;
+    this->member() = other.member();
     copy_all(other);
 }
 
@@ -1352,7 +1358,7 @@ basic_fields<Allocator>::
 swap(basic_fields& other, std::true_type)
 {
     using std::swap;
-    swap(alloc_, other.alloc_);
+    swap(this->member(), other.member());
     swap(set_, other.set_);
     swap(list_, other.list_);
     swap(method_, other.method_);
@@ -1365,7 +1371,7 @@ void
 basic_fields<Allocator>::
 swap(basic_fields& other, std::false_type)
 {
-    BOOST_ASSERT(alloc_ == other.alloc_);
+    BOOST_ASSERT(this->member() == other.member());
     using std::swap;
     swap(set_, other.set_);
     swap(list_, other.list_);

include/boost/beast/http/message.hpp

@@ -862,7 +862,8 @@ struct message
 #endif
     body()& noexcept
     {
-        return this->member();
+        return this->beast::detail::empty_base_optimization<
+            typename Body::value_type>::member();
     }
 
     /// Returns the body
@@ -873,7 +874,9 @@ struct message
 #endif
     body()&& noexcept
     {
-        return std::move(this->member());
+        return std::move(
+            this->beast::detail::empty_base_optimization<
+                typename Body::value_type>::member());
     }
 
     /// Returns the body
@@ -884,7 +887,8 @@ struct message
 #endif
     body() const& noexcept
     {
-        return this->member();
+        return this->beast::detail::empty_base_optimization<
+            typename Body::value_type>::member();
     }
 
 private: